eBulletin

Timely reminder about slow sluggish and stuck fermentations

Vintage 2013 will be remembered for one of the earliest and quickest vintages in recent times. With hot days followed by warm nights, grape sugar levels increased rapidly, culminating in a shorter vintage. With little to no rain in many regions since October 2012, berry nutrient content, especially YAN, is lower than recent vintages, due to reduced soil microbial activity. One of the challenges of a quick vintage can be vintage compression, and it is during these vintages that fermentation management becomes critical. A slow, sluggish or stuck ferment is not something that a winemaker needs or wants to deal with at any time, let alone in a vintage where compression sees logistical challenges such as tank space shortage.

Recent calls to the AWRI’s Winemaking and Extension Services team regarding slow and sluggish fermentations are a timely reminder about the importance of a number of critical factors that are essential in completing fermentations to dryness.

How do you know your ferment is slowing down or stuck? The best way is to monitor the fermentation twice daily for Baume or Brix, and temperature, and to plot this as a fermentation curve (a graph template is available for Australian winemaker use via password on the AWRI website). Graphing these measurements will quickly help identify when ferments begin to slow down, well before it has stopped, so that proactive action can be taken. The fermentation curves can also be used to establish if temperatures are becoming too cool or too warm for the ferment.

If the ferment is slowing down:

Try warming the ferment up if too cold, or cooling down if it’s getting hot. Yeast like the temperature to be kept relatively stable and, in general, do not function well at temperatures >32°C for reds or <15°C for whites.

If a natural ferment is slowing down, or a difficult ferment is suspected, then it is best to use an inoculated yeast.

If the yeast are not that active, or are settling to the bottom of the tank, try some agitation during the initial stage of the ferment; check for budding and/or viability by vital (e.g. methylene blue) staining.

Aerating once fermentation has started can also help the yeast build strong cell walls for ethanol tolerance to prevent the ferment becoming stuck; there is no risk of oxidising the wine while yeast are active. Aeration is a powerful fermentation stimulant when used correctly.

If the ferment is slow from the outset, make sure the ferment isn’t slowing due to any fermentation inhibitors including:

high sulfite concentrations in the must;

high volatile acidity from native microorganism growth/check for high bacteria count;

high Baume/Brix content. Some yeast are not tolerant to excessive sugar levels. Correspondingly some yeast don’t cope well in ethanol concentrations >15%;

agrochemical residues. Grapes harvested within withholding periods can contain some metals, sulfur or other compounds that can stress the fermentation yeast;

chlorine from water used during yeast hydration; and

very low pH (typically early harvest whites); some yeast cannot tolerate pH of 3.0 or less, especially when sulfite is present.

For some inhibitors such as high sulfite, a sacrificial yeast addition, or inoculation at twice the usual inoculum can be successful. Ensure the yeast have been strongly aerated to ensure aldehyde production which binds the excessive sulfite.

If the ferment is slow, or starting to produce sulfides, then nutrients might be an issue; particularly in highly clarified juice, or wines with some native microorganism growth. Measure the yeast assimilable nitrogen (YAN) level and make an addition if in the early stages of fermentation. If DAP doesn’t prevent sulfides, other nutrients such as vitamins or complex nutrients (e.g. inactive yeast products) can be tried. If sulfides persist, it might be coming from agrochemicals such as elemental sulfur.

Addition of yeast hulls, fresh yeast lees from a recently finished ferment, or addition to an active fermentation can help a struggling ferment go through to dryness.